West Nile virus is spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. After 10-14 days, the infected mosquito can transmit West Nile virus to people and animals through its bite. The virus is not spread from person-to-person or animal-to-person contact. Routine screening of blood donations since 2003 has greatly reduced the risk of West Nile virus infection through transfused blood.